In the past, many types of receivers for spent or fired shells have been disclosed. The more effective receivers were the type that required modifications to the firearms in order to mount them thereon. Other types were such that they could only be used on the particular weapons. Therefore, if one wanted to use a receiver on another type of weapon, inventive know-how was required, as evidenced by U.S. Pat. Nos. 2,354,277; 3,153,891; 3,156,991; and 3,618,458.